The present invention relates to an exposure control arrangement for a matrix-addressed imaging panel. More specifically, a matrix-addressed imaging panel using localized regions of capacitive coupling as a control, to infer an x-ray dose absorbed by an imaged subject to effect automatic exposure control (AEC).
Matrix-addressed x-ray imaging panels composed of semiconductor thin film transistors (TFTs) and radiation sensors have many useful applications in the fields of medicine and industrial inspection. Typically, such solid state imaging systems use a two dimensional matrix of radiation sensors and readout devices to convert radiation into digital signals corresponding to the incident radiant energy. In radiation imaging systems used in medical applications, radiation energy passing through, or emanating from, a patient""s body is detected and imaged.
When imaging energy from an imaging energy source is applied to a subject to be imaged, which may be a human or animal patient, or an inanimate part, portions of the imaging energy are blocked by the subject and remaining portions are transmitted through the subject and impinge on the imaging device. Typically this energy is converted to electrical charge or voltage either directly in an array of direct radiation sensors or indirectly using a scintillator which converts the energy to light which is converted to charge or voltage by an arrays of photo sensors such as a photodiodes. The amount of charge is advantageously proportional to the amount of absorbed radiation energy incident on the detector. Each sensor is connected via a corresponding controlled switch (e.g., TFT) to a corresponding data line. Each controlled switch is operable from a scan line, which is controlled by a scan line controller, to selectively pass an output signal from the corresponding sensor to its associated data line. The controlled switches are selectively closed, one at a time, to pass the output signals to the data lines. Each data line is read and integrated by a respective read-out amplifier. The separate integrated sensor values are processed and assembled to form an image representation of the subject when viewed on a display device.
It is desirable to have real time readout of the accumulated X-ray dose or signal in a panel of sensors sensing X-ray doses during exposure of an object (such as a person who is a medical patient) being imaged. Such a measure of accumulated dose is used to determine when the desired exposure level has been reached such that the X-ray tube can then be turned off. The desired exposure depends on factors such as the characteristics of the imaging system (e.g., need to avoid saturation), the characteristics of the object being imaged (e.g., in the case of a patient, the dose suitable under the best medical practice), or both factors. The method of turning off the X-ray tube based on the accumulated X-ray dose measurement is typically called automatic exposure control (AEC).
In one type of imaging system, the AEC includes one or more dedicated radiation sensors positioned below a detector (e.g., outside of an imaging panel) or film cassette. Signal generated by X-rays absorbed in the active volume of the sensor are used to infer the dose absorbed by the imaged object or by the imaging device. This method suffers from a number of disadvantages, including increased system cost and complexity (for the dedicated exposure control components), and the difficulty of calibrating the system (with the exposure control sensors separate from the imaging panel or film) to correlate the signal generated by the radiation sensor with the absorbed dose in the detector or object.
It is desirable to have a robust and effective system by which to sense imager panel saturation, and that system also desirably does not adversely affect the complexity, cost, or operating characteristics of the imaging panel. Such a system also desirably is readily calibrated.
It is further advantageous to use values representing a localized reading of instantaneous panel exposure to imaging radiation, as an input to a calculation of accumulated imaging energy dose, thus providing the basis for controlling total imaging radiation dose.
In one embodiment of this invention, an imaging system includes a matrix-addressed imaging panel having a plurality of pixels, each of which comprises a respective radiation sensor. Each pixel further comprises at least one respective pixel readout switch disposed to selectively couple the pixel radiation sensor to a respective data line in the imaging panel for purposes of reading the image information. At least one data line exposure signal, derived from at least one exposure-monitoring data line in the imaging panel, is coupled to an exposure controller. During this mode of operation the system is configured to sense a capacitively-coupled data line exposure signal from at least one radiation sensor capacitively-coupled to the exposure-monitoring data line. The data line exposure signals are processed by the exposure controller to provide a panel exposure signal corresponding to incident radiation sensed during an exposure control period when the respective pixel readout switches for the exposure-monitoring data line are in an electrically open condition.
In one embodiment of this invention, a method of operating an imaging system having a matrix-addressed imaging panel having a plurality of radiation sensor pixels, each pixel radiation sensor being connected via a respective pixel readout switch to a respective data line, includes the steps of: sensing exposure signals appearing on at least one data line from at least one radiation sensor with the pixel readout switches open; and generating a panel exposure signal based on the data line exposure signals generated during the periods when the pixel readout switches on a data line are in a non-conductive condition. The exposure signals that appear on the data lines via capacitive coupling of the photodiodes to the data lines and are sensed by an exposure controller configured to generate a real-time panel exposure signal that typically is used as a control input for the imaging energy source.
In another embodiment of the invention, a method and system for an exposure control arrangement for a matrix-addressed imaging panel comprises at least one Automatic Exposure Control (AEC) signal line disposed in a respective localized region of an imaging panel so as to provide a respective AEC receptive field region defined by the pixel diode electrodes which are disposed vertically adjacent to the AEC electrode. The AEC line in turn is coupled to an external amplifier to generate a localized exposure signal for use with panel exposure control.
In a still further embodiment of this invention, an imager comprises both the coupling for the data line exposure signal and also at least one AEC electrode as described above.